History is Written by the Survivors

In the theory of evolution, fitness is not merely about survival; a species also has to reproduce. Bacteria that express a drug-resistant trait may outlive an exposure, but if this trait slows the growth of the cells, they may not be able to divide quickly, putting them at a disadvantage once the drug is removed. The fittest organisms therefore adopt traits that allow a trade-off between survival of an environmental stress and an ability to reproduce [1]. Evolutionary biologists quantify this fitness with a mathematical “reproductive function,” which depends on a selected trait (such as age) and whose height gives the probability that a species with that trait will survive and reproduce. This function can, for example, provide a useful metric in drug trials, and it could help evaluate the effects of environmental stresses—such as changing food supply or human intervention—on plants and animals. But determining it entails following thousands of individuals and monitoring their every reproduction and death event for extended periods of time. Guillaume Lambert from the University of Chicago and Edo Kussell from New York University demonstrate with experiments on bacteria that the same information can be obtained from a single surviving-cell lineage, the chain of offspring from an ancestor to a surviving cell [2]. This approach would make it possible to quickly assess, with far fewer measurements, the effects of an environmental stress in experimental trials.